The Galls of Essex. 99
Galls are known to occur on plants in nearly every order
of the vegetable kingdom; they vary greatly in their form
and in their situation on the plant. This will be fully
exemplified presently, when I shall come to a detailed
examination of our Essex gall fauna.
More strictly speaking, the term "gall" is applied to an
abnormal vegetable growth. Lacaze Duthiers defines "galls"
as comprising "all abnormal vegetable productions developed
on plants by the action of animals, more particularly by
insects, whatever may be their form, size, or situation."2
This is by no means a scientific definition, but the limitation
is convenient and the intention is apparent. It must,
however, be remembered that all abnormal vegetable growths
are not galls, as will readily be seen from a study of Dr.
Masters's 'Vegetable Teratology.'3
This brings us to a starting point—the cause of galls. A
gall is an abnormal vegetable growth produced through the
internal action of some foreign agency. This agency is
generally either the influence of a fungus or of some
Arthropod. Certain of the Anguillulidae,4 belonging to the
Nematode worms, and some Infusoria (Vibrio, &c), are also
gall-makers, and possibly even certain Rotifera. Both the
animal and the vegetable causative actions produce often
remarkably similar inflammatory and stimulated growths,
and most frequently these are abundantly distinct from any
normal production or growth of the galled object.
So far the subject is clear, but the exact way in which the
foreign influence is brought to bear on the vegetable growth
is still matter for conjecture and experiment. The actual
progressive structure of the gall itself also still affords a good
working field for the structural botanist.
The "gallic" force of an insect, so to speak, is vulgarly
2 'Annales des Sciences naturelles.' Botanique. 3rd series, vol. xix.,
p. 273. Paris, 1853.
3 'Vegetable Teratology.' By Maxwell T. Masters, M.D., F.L.S.
London (Ray Society), 1869.
4 'Monograph on the Anguillulidae.' By H. Charlton Bastian, M.A.,
M.B. Transactions of the Linnean Society, vol. xxv., pp. 73—184,
pls. 9—13. (1866).